Solid-state CO2 sensors comprising O-2, LiFeO2-LiFe5O8/Li+ conducting glass ceramics/Li2CO3-Li3PO4/Au,CO2, O-2 have been developed, where Li2CO3 acts as a lithium-ion conductor and a sensing electrode material, with the two-phase mixture LiFeO2-LiFe5O8 as the reference electrode. The electromotive force (emf) of the sensor responded well to changes of CO, concentration at temperatures around 500 degreesC, while it is independent of the oxygen partial pressure. Its long-term stability is good. the emf of the sensor was reproducible within +/-3 mV over than I year in an environment of 95% humidity, This result shows that the developed CO2 sensors have high humidity resistance and good stability. The sensing mechanism is also discussed,